The present invention relates to multi-antenna transmission schemes for wireless communication networks and more particularly, to an arrangement and a method for adapting between transmitter and receiver configurations responsive to channel conditions and other factors.
In most conventional second generation (2G) and third generation (3G) wireless communication systems, both the transmitter and receiver are equipped with a single antenna. Such systems are known as single input, single output (SISO) systems. In recent years, researchers have proposed using multiple antennas at the transmitter and/or receiver to improve performance of the communication system. Transmit diversity is one example of a multiple antenna transmission scheme that has been proposed as a way of improving the signal-to-interference ratio (SINR) at the receiver. As used herein, SINR includes noise interference. Transmit diversity systems are also known as multiple input, single output (MISO) systems. Transmit diversity or MISO systems employ multiple antennas at the transmitter to transmit a signal to the receiver. The signals from each transmit antenna arrive at the receiver through different propagation channels. The receiver can select the “best” signal (typically the one with the highest SINR), or can combine multiple signals to achieve a higher SINR through combining gain. Transmit diversity using space-time codes further improves the SINR by coding the transmit signal prior to transmission. While transmit diversity schemes realize improvements in the SINR, peak rate improvement is limited to single-antenna modulation and coding options.
Multiple input, multiple output (MIMO) systems employ multiple antennas at the receiver as well as at the transmitter. MIMO systems exploit the spatial dimensions of the signal at the receiver to achieve higher spectral efficiency and higher data rates without increasing bandwidth. The trade-off results in greater complexity of the transmitter and receiver. MIMO systems can be broadly classified as limited feedback and rich feedback systems. In limited feedback MIMO systems, only limited information concerning the communication channel is fed back from the receiver to the transmitter. Limited feedback systems improve the attainable peak transfer rate and are more advantageous in high SINR environments. In rich feedback systems, the receiver sends channel coefficients, or other information from which channel coefficients can be estimated, back to the transmitter. The transmitter can use the knowledge of the channel to filter the transmit signal prior to transmission. Rich feedback systems provide additional benefit due to array gain from multiple transmit antennas, which may be substantial for certain array configurations.
Unfortunately, there is no one system configuration that performs consistently better than the others over the range of possible operating conditions envisioned for deployment. Some of the operating conditions that affect performance include the expected operating SINR at the receiver; the propagation channel and interference environment observed at the receiver; the amount of channel feedback allowed; the assumed user data traffic; and the number of desired allowable transmit and receive antennas. What is desired is to have a robust approach that works well for different antenna configurations in a wide range of operating conditions.